The Queen's swan warden, Professor Christopher Perrins of Oxford University, weighs the birds and checks them for fishhooks.

We then tag them with small, stainless steel leg rings - marking the beaks went out about three years ago. The rings aren't so traditional, but do provide more information.

Conservation role

For centuries, the swan marker was a sort of gamekeeper; when I took on the role seven years ago, I saw the job as having more to offer conservation and children's education.

David Barber: "I've always worked on the river"

I'd been a swan upper for many years, and my name was put forward when my predecessor, Captain John Turk, retired at 82.

Although I'd never intended to have the job, I realised that this was an opportunity to help the river and to help people better understand what we do. That's how I got the job, I think - because I wanted to change it.

I'll hold the post, health permitting, until I'm 70. It's called part-time, but I deal with swan problems day in and day out, and answer questions from all over the world.

Respect the river

We have an asset here in the Thames, yet the river is a little in decline at the moment. People put all this concrete and steel sheeting along the banks, so the reed beds disappear.

A breeding pair nest near Cookham

If this continues, what's going to be left in 50 or 100 years time? We risk losing the habitats of not only swans, but all the wildlife in the area.

I want to encourage young children to respect the river, respect wildlife.

We take a few school groups out on a passenger boat during Swan Upping, and they get to see exactly what's going on. Then later, if they take up fishing, they understand that discarded fishing tackle can be dangerous to wildlife.

In the 1980s, swan numbers on the Thames had dwindled by two-thirds due to lead poisoning from fishing weights. Once these were banned, numbers built up from 400 to between 1,000 and 1,200.